A piston-type compressor may include a cylinder block C having formed therein cylinders B through which pistons A slide reciprocally and a head E mounted at the cylinder block C via a valve plate D, as shown in FIG. 4. At the valve plate D in FIG. 4, a suction port G, with an open end thereof located on the cylinder block side opened/closed with a suction valve F, and a discharge port I, with an open end thereof located on the head side opened/closed with a discharge valve H, are formed in correspondence to each of the cylinders B.
The suction valve F and the discharge valve H, which are normally reed valves constituted with thin-plate springs having base portions thereof fixed onto the valve plate D, open/close the exit-side ends of the corresponding ports respectively in correspondence to the pressure difference between the compression chamber J and a suction chamber K and in correspondence to the pressure difference between the compression chamber J and a discharge chamber L.
As shown in the figure, discharge ports I in the related art are usually formed so as to sustain a uniform sectional shape remaining unchanged from the entrance end through the exit and (a circular section with a specific diameter). Over this area where the uniform sectional shape is sustained, a condition referred to as “dead volume” tends to occur, whereby a compressed fluid stays back instead of being let out. In addition, in a compressor assuming such a valve structure, in which the operating fluid having traveled to the discharge valve H is let out through the gap formed between the discharge valve H and the valve plate D, is prone to pressure loss. Further issues inherent to this type of compressor include surface tension attributable to the lubricating oil causing the discharge valve H to open with a delay and a lowered volumetric efficiency occurring as the residual compressed fluid remaining inside the discharge port I becomes re-expanded during a suction stroke.
The extent of loss attributable to the re-expansion may be reduced by reducing the diameter of the discharge port. However, when the discharge port assumes a smaller circumference and a smaller sectional area, the extent of pressure loss is bound to be more significant and the valve is most likely to open with a greater delay. If the diameter of the discharge port and, consequently, the circumference and the sectional area of the discharge port are increased, the pressure loss and the valve opening delay are both decreased. However, the presence of the residual compressed fluid (dead volume) in the discharge port is bound to be more significant.
In an attempt to address the issues discussed above, the compressor disclosed in patent reference literature 1 in the related art assumes a structure that assures a smooth flow of coolant and thus reduces the extent of pressure loss by forming at a discharge port at a valve plate of the compressor with an exit-side straight portion formed adjacent to an exit end of the discharge port and assuming a cylindrical shape with a uniform diameter and an entrance-side straight portion formed adjacent to an entrance end of the discharge port and assuming a cylindrical shape with a uniform diameter set smaller than the diameter of the exit-side straight portion and by forming between the exit-side straight portion and the entrance-side straight portion an enlarged diameter portion in a funnel shape, the diameter of which increases gradually toward the exit-side straight portion.
Patent reference literature 1: Japanese Unexamined Patent Publication No. 2003-1390633